[Milsurplus] FET Regulator: How It Do Dat?

[David Stinson]

Dear Smart People:
Here's a FET High-voltage regulator that works:

https://goo.gl/photos/jbVaywJcWGTNWkbq9
There's a bunch of "better ways" and improvements
etc., but please- just want to address this circuit.

I understand how the zener stack must be in 
conduction and you pick a point in the 
stack for the reference voltage on the gate.
If the input Voltage ever falls below the value
needed to keep the zeners conducting,
the FET shuts off.

What I don't understand is exactly how 
the FET matches the Gate Voltage and 
the Source Voltage, thus regulating.  
It seems to me that the 200V clamped 
down to 18V at the Gate should turn the 
thing full on and dump the input voltage
 to the output with no regulation.

Excuse my dummy-ness; how does the
FET regulate?

Thanks.